Cellular telephones are commonplace in today's society. As a result, there is a desire to be able to accommodate more users at higher data rates in a given geographic area. Wireless communications are also being implemented for services besides voice. For example, wireless communications are now being planned and implemented to allow access to the Internet and other networks from mobile devices.
As the uses for wireless communications continue to expand, the reliability of the communication must be continually improved. One problem that greatly degrades the performance of mobile communications systems is multipath fading. This phenomenon is caused by the reflection of the signal from multiple scatterers. Moreover, this environment is constantly changing due to the motion of mobile receivers and transmitters.
One technique that has been used in practice to combat fading is diversity. Diversity implies the exploitation of multiple copies of the transmitted signal at the receiver. Common examples of diversity includes, time diversity, frequency (Doppler), and spatial diversity. The mobile wireless channel provides inherent diversity in the form of multiple paths (time diversity) which is exploited in current cellular systems through the use of rake receivers.
In current systems, rake receivers exploit multipath diversity by combining the various multipath components. The use of rake receivers, however, assumes that the channel characteristics vary slowly over time and is suitable for slow-fading situations. As a result, the performance degrades in the presence of Doppler induced by rapid channel variations due to fast relative motion between the transmitter and the receiver.
Recently a new technique to exploit rapid channel variations to improve performance through joint multipath-Doppler diversity processing has been proposed, in the research community, for CDMA systems. This technique facilitates maximal exploitation of channel diversity in slow as well as fast fading scenarios, thus combating fading in both situations.